Does anyone know of a good system for booking shared netbooks?

We are an elementary school with 28 new netbooks in our library. We would like teachers to borrow them flexibly as needed, in any combination from 1 to 28 at a time. We have an 8 period day. Does anyone have a good system for pre-booking variable groups of netbooks to make sure that they are available when teachers want to use them? Any comments or suggestions would be welcome.